The present invention relates to a pulse width modulation amplifier circuit, and particularly relates to a pulse width modulation amplifier circuit having a reduced degree of disturbance exerted onto other equipment by a high-frequency carrier signal to be modulated into a pulse width modulation signal with an input analog signal.
One type of pulse width modulation amplifier (PWM amplifier), is configured so that a high-frequency triangular wave carrier signal is modulated with an analog signal, for example, an audio signal or the like, so as to be converted into a pulse width modulation signal. The pulse width modulation signal is power-amplified and then demodulated by removing the carrier signal with a filter. The demodulated signal is applied to a load such as a speaker or the like. Recently, PWM amplifiers of this type have been used in mobile audio equipment or the like because of the excellent efficiency of PWM amplifiers in achieving power amplification.
FIG. 3 shows an example of a conventional PWM amplifier. The reference numeral 1 designates an input terminal for receiving an analog signal. The analog signal is applied to an inverted input terminal of a comparator 2. The high-frequency output of a triangular wave carrier oscillator 3, on the other hand, is applied to a non-inverted input terminal of the comparator 2, so that the carrier signal is modulated with the analog signal so as to be converted into a pulse width modulation signal. The pulse width modulation signal obtained from the comparator 2 is passed through a drive amplifier 4, and then amplified by a pulse amplifier (power amplifier) 5 consisting of MOS power FETs. After the carrier signal is eliminated from the amplified pulse width modulation signal through a filter circuit consisting of a choke coil 6 and a capacitor 7, the demodulated signal drives a speaker 9 or the like connected to an output terminal 8.
In the above configuration, a high-frequency signal having a frequency of about 200 KHz, for example, is used as a triangular wave carrier signal to be modulated with an input analog signal into a pulse width modulation signal. The carrier signal has a disadvantage in that it contains higher harmonic components, and the harmonic components as well as the fundamental wave of the carrier signal are radiated, thereby causing various disturbances to radio receivers or the like.
In order to reduce such disturbances, it has been proposed that the fundamental wave of about 200 KHz be frequency-modulated with a specific frequency, for example, of about 20 Hz, to thereby disperse the carrier signal energy to reduce the levels of the radiated fundamental wave and its harmonic components to a certain extent.
However, the result of such dispersal of carrier signal energy does not extinguish the high-frequency signal, but simply reduces it, and the disturbance to receivers still occurs, though to a reduced extent as compared to the case where no countermeasures are taken. A further problem is that the 20 Hz signal and its harmonic components are demodulated and reproduced.